GeoTIFF geographic grid alignment

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GeoTIFF geographic grid alignment

tjn98
GeoTIFF geographic grid alignment Dear All,

  I’m looking for some guidance on the correct interpretation of
GeoTIFF raster spaces, in particular their correct alignments at the
half-pixel level. Judging by the number of mutually contradictory
examples I’ve found, this is a common source of confusion.

  My interpretation of the rather terse text in the GeoTIFF Format
Specification (Revision 1.0) is that, for the example of a global
map measuring 129,600 longitude pixels by 64,800 latitude pixels,
a “PixelIsArea” raster described by:

    ModelTiepointTag = (0.0, 0.0, 0.0, -180.0, 90.0, 0.0)
    ModelPixelScaleTag = (0.002777777778, 0.002777777778, 0.0)
    GeoKeyDirectoryTag:
        GTModelTypeGeoKey    = 2    (ModelTypeGeographic)
        GTRasterTypeGeoKey   = 1    (RasterPixelIsArea)
        GeographicTypeGeoKey = 4326 (GCS_WGS_84)

exactly fits into a –180 -> 180 by –90 -> 90 degree box that touches
the edges of the extreme cell boundaries, and that a “PixelIsPoint”
raster, whose entries fall exactly in the centre of the “PixelIsArea”
cells in the first example is described by:

    ModelTiepointTag = (-0.5, -0.5, 0.0, -180.0, 90.0, 0.0)
    ModelPixelScaleTag = (0.002777777778, 0.002777777778, 0.0)
    GeoKeyDirectoryTag:
        GTModelTypeGeoKey    = 2    (ModelTypeGeographic)
        GTRasterTypeGeoKey   = 2    (RasterPixelIsPoint)
        GeographicTypeGeoKey = 4326 (GCS_WGS_84)

  Variants I have seen include both +0.5 and –0.5 pixel offsets for
the PixelIsArea case, negative ModelPixelScaleTag values for the
“J” direction and 0.0 pixel offsets for the PixelIsPoint case. QGIS,
for example, appears to assume the last.

  Can anyone help?

    Many thanks,

      Tim.

------------------------------------------------------------------------
 Dr T.J. Nightingale
  Space Science and Technology Department
   Rutherford Appleton Laboratory
    Chilton, Didcot                   Phone: +44/0 1235 445914
     Oxon OX11 0QX                    Fax:   +44/0 1235 445848
      United Kingdom                  Email: tim.nightingale@...
------------------------------------------------------------------------



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Re: GeoTIFF geographic grid alignment

David Burken
Hi,

If RasterPixelIsPoint I think it would be (0.5, -0.5).

Dave

----- Original Message -----
From: "tjn98" <[hidden email]>
To: [hidden email]
Sent: Monday, May 10, 2010 10:28:07 AM GMT -05:00 US/Canada Eastern
Subject: [Geotiff] GeoTIFF geographic grid alignment

GeoTIFF geographic grid alignment Dear All,

  I’m looking for some guidance on the correct interpretation of
GeoTIFF raster spaces, in particular their correct alignments at the
half-pixel level. Judging by the number of mutually contradictory
examples I’ve found, this is a common source of confusion.

  My interpretation of the rather terse text in the GeoTIFF Format
Specification (Revision 1.0) is that, for the example of a global
map measuring 129,600 longitude pixels by 64,800 latitude pixels,
a “PixelIsArea” raster described by:

    ModelTiepointTag = (0.0, 0.0, 0.0, -180.0, 90.0, 0.0)
    ModelPixelScaleTag = (0.002777777778, 0.002777777778, 0.0)
    GeoKeyDirectoryTag:
        GTModelTypeGeoKey    = 2    (ModelTypeGeographic)
        GTRasterTypeGeoKey   = 1    (RasterPixelIsArea)
        GeographicTypeGeoKey = 4326 (GCS_WGS_84)

exactly fits into a –180 -> 180 by –90 -> 90 degree box that touches
the edges of the extreme cell boundaries, and that a “PixelIsPoint”
raster, whose entries fall exactly in the centre of the “PixelIsArea”
cells in the first example is described by:

    ModelTiepointTag = (-0.5, -0.5, 0.0, -180.0, 90.0, 0.0)
    ModelPixelScaleTag = (0.002777777778, 0.002777777778, 0.0)
    GeoKeyDirectoryTag:
        GTModelTypeGeoKey    = 2    (ModelTypeGeographic)
        GTRasterTypeGeoKey   = 2    (RasterPixelIsPoint)
        GeographicTypeGeoKey = 4326 (GCS_WGS_84)

  Variants I have seen include both +0.5 and –0.5 pixel offsets for
the PixelIsArea case, negative ModelPixelScaleTag values for the
“J” direction and 0.0 pixel offsets for the PixelIsPoint case. QGIS,
for example, appears to assume the last.

  Can anyone help?

    Many thanks,

      Tim.

------------------------------------------------------------------------
 Dr T.J. Nightingale
  Space Science and Technology Department
   Rutherford Appleton Laboratory
    Chilton, Didcot                   Phone: +44/0 1235 445914
     Oxon OX11 0QX                    Fax:   +44/0 1235 445848
      United Kingdom                  Email: tim.nightingale@...
------------------------------------------------------------------------



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Re: GeoTIFF geographic grid alignment

Max Martinez
GeoTIFF geographic grid alignment

Tim,

 

I think the GeoTIFF spec tries to show through its examples that although the orientation of the positive J raster space axis is usually in the opposite direction of the positive Y model space axis, it is expected that both pixel scales will be positive under these conditions. (They probably could have been clearer if they described full map extents of the images in the examples). Negation should be used appropriately as the current condition differs from that usual condition. This is also supported by the example on page 27 where the ModelTransformationTag content equivalents of an image with tiepoint (I,J,K,X,Y,Z) and scale (Sx, Sy, Sz) is provided. So I think you have that right in your example.

 

We use a -0.5, -0.5 translation to translate pixel is area to pixel is point just has you have done below.

 

Max


From: [hidden email] [mailto:[hidden email]] On Behalf Of [hidden email]
Sent: Monday, May 10, 2010 1:48 PM
To: tjn98
Cc: [hidden email]
Subject: Re: [Geotiff] GeoTIFF geographic grid alignment

 

Hi,

If
RasterPixelIsPoint I think it would be (0.5, -0.5).

Dave

----- Original Message -----
From: "tjn98" <[hidden email]>
To: [hidden email]
Sent: Monday, May 10, 2010 10:28:07 AM GMT -05:00 US/Canada Eastern
Subject: [Geotiff] GeoTIFF geographic grid alignment

Dear All,

  I’m looking for some guidance on the correct interpretation of
GeoTIFF raster spaces, in particular their correct alignments at the
half-pixel level. Judging by the number of mutually contradictory
examples I’ve found, this is a common source of confusion.

  My interpretation of the rather terse text in the GeoTIFF Format
Specification (Revision 1.0) is that, for the example of a global
map measuring 129,600 longitude pixels by 64,800 latitude pixels,
a “PixelIsArea” raster described by:

    ModelTiepointTag = (0.0, 0.0, 0.0, -180.0, 90.0, 0.0)
    ModelPixelScaleTag = (0.002777777778, 0.002777777778, 0.0)
    GeoKeyDirectoryTag:
        GTModelTypeGeoKey    = 2    (ModelTypeGeographic)
        GTRasterTypeGeoKey   = 1    (RasterPixelIsArea)
        GeographicTypeGeoKey = 4326 (GCS_WGS_84)

exactly fits into a –180 -> 180 by –90 -> 90 degree box that touches
the edges of the extreme cell boundaries, and that a “PixelIsPoint”
raster, whose entries fall exactly in the centre of the “PixelIsArea”
cells in the first example is described by:

    ModelTiepointTag = (-0.5, -0.5, 0.0, -180.0, 90.0, 0.0)
    ModelPixelScaleTag = (0.002777777778, 0.002777777778, 0.0)
    GeoKeyDirectoryTag:
        GTModelTypeGeoKey    = 2    (ModelTypeGeographic)
        GTRasterTypeGeoKey   = 2    (RasterPixelIsPoint)
        GeographicTypeGeoKey = 4326 (GCS_WGS_84)

  Variants I have seen include both +0.5 and –0.5 pixel offsets for
the PixelIsArea case, negative ModelPixelScaleTag values for the
“J” direction and 0.0 pixel offsets for the PixelIsPoint case. QGIS,
for example, appears to assume the last.

  Can anyone help?

    Many thanks,

      Tim.

------------------------------------------------------------------------
 Dr T.J. Nightingale
  Space Science and Technology Department
   Rutherford Appleton Laboratory
    Chilton, Didcot                   Phone: +44/0 1235 445914
     Oxon OX11 0QX                    Fax:   +44/0 1235 445848
      United Kingdom                  Email: tim.nightingale@...
------------------------------------------------------------------------


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Re: GeoTIFF geographic grid alignment

David Burken
Max, Tim,

If your tie upper left corner of the pixel (AREA) and relative to an Easting Northing of (10,000, 10,000) and your pixel scale is (100, 100) meters then to convert to point (center) it would be:

Move the Easting 50 meters to the east or right ( +.5 pixel)

Move the Northing 50 meters to the South or down ( -.5 pixel)

So doing ( -.5, -5 ) is wrong in a map projection.

We actually did a drawing of this on ossim.org to show the error when ignored but I cannot find it. Sorry.

Hope that helps,
Dave


On 05/13/2010 05:15 PM, Max Martinez wrote:
GeoTIFF geographic grid alignment

Tim,

 

I think the GeoTIFF spec tries to show through its examples that although the orientation of the positive J raster space axis is usually in the opposite direction of the positive Y model space axis, it is expected that both pixel scales will be positive under these conditions. (They probably could have been clearer if they described full map extents of the images in the examples). Negation should be used appropriately as the current condition differs from that usual condition. This is also supported by the example on page 27 where the ModelTransformationTag content equivalents of an image with tiepoint (I,J,K,X,Y,Z) and scale (Sx, Sy, Sz) is provided. So I think you have that right in your example.

 

We use a -0.5, -0.5 translation to translate pixel is area to pixel is point just has you have done below.

 

Max


From: [hidden email] [[hidden email]] On Behalf Of [hidden email]
Sent: Monday, May 10, 2010 1:48 PM
To: tjn98
Cc: [hidden email]
Subject: Re: [Geotiff] GeoTIFF geographic grid alignment

 

Hi,

If
RasterPixelIsPoint I think it would be (0.5, -0.5).

Dave

----- Original Message -----
From: "tjn98" [hidden email]
To: [hidden email]
Sent: Monday, May 10, 2010 10:28:07 AM GMT -05:00 US/Canada Eastern
Subject: [Geotiff] GeoTIFF geographic grid alignment

Dear All,

  I’m looking for some guidance on the correct interpretation of
GeoTIFF raster spaces, in particular their correct alignments at the
half-pixel level. Judging by the number of mutually contradictory
examples I’ve found, this is a common source of confusion.

  My interpretation of the rather terse text in the GeoTIFF Format
Specification (Revision 1.0) is that, for the example of a global
map measuring 129,600 longitude pixels by 64,800 latitude pixels,
a “PixelIsArea” raster described by:

    ModelTiepointTag = (0.0, 0.0, 0.0, -180.0, 90.0, 0.0)
    ModelPixelScaleTag = (0.002777777778, 0.002777777778, 0.0)
    GeoKeyDirectoryTag:
        GTModelTypeGeoKey    = 2    (ModelTypeGeographic)
        GTRasterTypeGeoKey   = 1    (RasterPixelIsArea)
        GeographicTypeGeoKey = 4326 (GCS_WGS_84)

exactly fits into a –180 -> 180 by –90 -> 90 degree box that touches
the edges of the extreme cell boundaries, and that a “PixelIsPoint”
raster, whose entries fall exactly in the centre of the “PixelIsArea”
cells in the first example is described by:

    ModelTiepointTag = (-0.5, -0.5, 0.0, -180.0, 90.0, 0.0)
    ModelPixelScaleTag = (0.002777777778, 0.002777777778, 0.0)
    GeoKeyDirectoryTag:
        GTModelTypeGeoKey    = 2    (ModelTypeGeographic)
        GTRasterTypeGeoKey   = 2    (RasterPixelIsPoint)
        GeographicTypeGeoKey = 4326 (GCS_WGS_84)

  Variants I have seen include both +0.5 and –0.5 pixel offsets for
the PixelIsArea case, negative ModelPixelScaleTag values for the
“J” direction and 0.0 pixel offsets for the PixelIsPoint case. QGIS,
for example, appears to assume the last.

  Can anyone help?

    Many thanks,

      Tim.

------------------------------------------------------------------------
 Dr T.J. Nightingale
  Space Science and Technology Department
   Rutherford Appleton Laboratory
    Chilton, Didcot                   Phone: +44/0 1235 445914
     Oxon OX11 0QX                    Fax:   +44/0 1235 445848
      United Kingdom                  Email: tim.nightingale@...
------------------------------------------------------------------------


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Re: GeoTIFF geographic grid alignment

tjn98
Re: [Geotiff] GeoTIFF geographic grid alignment Dear Max, David,

  Thanks for getting back to me. I see I’m not alone in my confusion.
I think we all agree that ModelTiePointTag 0.5 offsets (of whatever
sign) ARE required for RasterPixelIsPoint and ARE NOT required for
RasterPixelIsArea. This is a big step forward for me!

  We also agree that the ModelPixelScaleTag entries should be positive
(though I think this is inconsistent with the left-handed I/J system
and right-handed lon/lat system).

  I would have thought that the offset sign would have to be the same for
both the I and J directions (and K for that matter) as the handedness
of the IJK axis system doesn’t have any effect on this. Even though J
is positive down, the edge of the cell centred on the first point still
has to start “before” the beginning of the axis, as would be the case
for I also. Consequently, I’d think both must be negative.

  If the offsets were applied instead to the geographic tie point
information, then one adjustment would have to be positive (Sx) and
the other negative (Sy).

  How does this sound?

    Tim.

------------------------------------------------------------------------
 Tim Nightingale
  Space Science and Technology Department
   Rutherford Appleton Laboratory
    Chilton, Didcot                   Phone: +44/0 1235 445914
     Oxon OX11 0QX                    Fax:   +44/0 1235 445848
      United Kingdom                  Email: tim.nightingale@...
------------------------------------------------------------------------


 


On 14/05/2010 09:03, "David Burken" <dburken@...> wrote:

Max, Tim,

If your tie upper left corner of the pixel (AREA) and relative to an Easting Northing of (10,000, 10,000) and your pixel scale is (100, 100) meters then to convert to point (center) it would be:

Move the Easting 50 meters to the east or right ( +.5 pixel)

Move the Northing 50 meters to the South or down ( -.5 pixel)

So doing ( -.5, -5 ) is wrong in a map projection.

We actually did a drawing of this on ossim.org to show the error when ignored but I cannot find it. Sorry.

Hope that helps,
Dave


On 05/13/2010 05:15 PM, Max Martinez wrote:
  GeoTIFF geographic grid alignment    
 

Tim,



I think the GeoTIFF spec tries to show through its examples that although the orientation of the positive J raster space axis is usually in the opposite direction of the positive Y model space axis, it is expected that both pixel scales will be positive under these conditions. (They probably could have been clearer if they described full map extents of the images in the examples). Negation should be used appropriately as the current condition differs from that usual condition. This is also supported by the example on page 27 where the ModelTransformationTag content equivalents of an image with tiepoint (I,J,K,X,Y,Z) and scale (Sx, Sy, Sz) is provided. So I think you have that right in your example.



We use a -0.5, -0.5 translation to translate pixel is area to pixel is point just has you have done below.



Max

 





From: geotiff-bounces@... [[hidden email]] On Behalf Of dburken@...
 Sent: Monday, May 10, 2010 1:48 PM
 To: tjn98
 Cc: Geotiff@...
 Subject: Re: [Geotiff] GeoTIFF geographic grid alignment

 


 

Hi,
 
If
RasterPixelIsPoint I think it would be (0.5, -0.5).
 
Dave
 
 
----- Original Message -----
From: "tjn98" <tim.nightingale@...> <[hidden email]>
To: Geotiff@...
Sent: Monday, May 10, 2010 10:28:07 AM GMT -05:00 US/Canada Eastern
Subject: [Geotiff] GeoTIFF geographic grid alignment
 
 
Dear All,
 
  I’m looking for some guidance on the correct interpretation of
GeoTIFF raster spaces, in particular their correct alignments at the
half-pixel level. Judging by the number of mutually contradictory
examples I’ve found, this is a common source of confusion.
 
  My interpretation of the rather terse text in the GeoTIFF Format
Specification (Revision 1.0) is that, for the example of a global
map measuring 129,600 longitude pixels by 64,800 latitude pixels,
a “PixelIsArea” raster described by:
 
    ModelTiepointTag = (0.0, 0.0, 0.0, -180.0, 90.0, 0.0)
    ModelPixelScaleTag = (0.002777777778, 0.002777777778, 0.0)
    GeoKeyDirectoryTag:
        GTModelTypeGeoKey    = 2    (ModelTypeGeographic)
        GTRasterTypeGeoKey   = 1    (RasterPixelIsArea)
        GeographicTypeGeoKey = 4326 (GCS_WGS_84)
 
exactly fits into a –180 -> 180 by –90 -> 90 degree box that touches
the edges of the extreme cell boundaries, and that a “PixelIsPoint”
raster, whose entries fall exactly in the centre of the “PixelIsArea”
cells in the first example is described by:
 
    ModelTiepointTag = (-0.5, -0.5, 0.0, -180.0, 90.0, 0.0)
    ModelPixelScaleTag = (0.002777777778, 0.002777777778, 0.0)
    GeoKeyDirectoryTag:
        GTModelTypeGeoKey    = 2    (ModelTypeGeographic)
        GTRasterTypeGeoKey   = 2    (RasterPixelIsPoint)
        GeographicTypeGeoKey = 4326 (GCS_WGS_84)
 
  Variants I have seen include both +0.5 and –0.5 pixel offsets for
the PixelIsArea case, negative ModelPixelScaleTag values for the
“J” direction and 0.0 pixel offsets for the PixelIsPoint case. QGIS,
for example, appears to assume the last.
 
  Can anyone help?
 
    Many thanks,
 
      Tim.
 
------------------------------------------------------------------------
 Dr T.J. Nightingale
  Space Science and Technology Department
   Rutherford Appleton Laboratory
    Chilton, Didcot                   Phone: +44/0 1235 445914
     Oxon OX11 0QX                    Fax:   +44/0 1235 445848
      United Kingdom                  Email: tim.nightingale@...
------------------------------------------------------------------------
 
 
 
 


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Re: GeoTIFF geographic grid alignment

David Burken
Max, see below:

On 05/14/2010 06:12 AM, tjn98 wrote:
Re: [Geotiff] GeoTIFF geographic grid alignment Dear Max, David,

  Thanks for getting back to me. I see I’m not alone in my confusion.
I think we all agree that ModelTiePointTag 0.5 offsets (of whatever
sign) ARE required for RasterPixelIsPoint and ARE NOT required for
RasterPixelIsArea. This is a big step forward for me!

You can have a tie of 0,0 or any other value and be point or area.  The point/area just tells the application that the tie is relative to the center(point) or upper left.

So given scale of 50,50 then:
Area tie of (0, 0) (10000, 10000) is the same as:
Point tie of (0, 0) (10050, 9950)

Here are two images one point and one area.  They contain the exact same image data and should swipe perfectly. 

$ ossim-info -d area.tif | grep tie
tiff.image0.model_tie_point: 0.000000000000000 0.000000000000000 0.000000000000000 537855.000000000000000 3964495.000000000000000 0.000000000000000

$ ossim-info -d point.tif | grep tie
tiff.image0.model_tie_point: 0.000000000000000 0.000000000000000 0.000000000000000 537860.000000000000000 3964490.000000000000000 0.000000000000000

$ ossim-info -d point.tif | grep scale
tiff.image0.model_pixel_scale: 10.000000000000000 10.000000000000000 0.000000000000000

$ ossim-info -d area.tif | grep scale
tiff.image0.model_pixel_scale: 10.000000000000000 10.000000000000000 0.000000000000000

Hope that helps,
Dave


  We also agree that the ModelPixelScaleTag entries should be positive
(though I think this is inconsistent with the left-handed I/J system
and right-handed lon/lat system).

Yes positive
  I would have thought that the offset sign would have to be the same for
both the I and J directions (and K for that matter) as the handedness
of the IJK axis system doesn’t have any effect on this. Even though J
is positive down, the edge of the cell centred on the first point still
has to start “before” the beginning of the axis, as would be the case
for I also. Consequently, I’d think both must be negative.

  If the offsets were applied instead to the geographic tie point
information, then one adjustment would have to be positive (Sx) and
the other negative (Sy).

  How does this sound?

    Tim.

------------------------------------------------------------------------
 Tim Nightingale
  Space Science and Technology Department
   Rutherford Appleton Laboratory
    Chilton, Didcot                   Phone: +44/0 1235 445914
     Oxon OX11 0QX                    Fax:   +44/0 1235 445848
      United Kingdom                  Email: tim.nightingale@...
------------------------------------------------------------------------


 


On 14/05/2010 09:03, "David Burken" <dburken@...> wrote:

Max, Tim,

If your tie upper left corner of the pixel (AREA) and relative to an Easting Northing of (10,000, 10,000) and your pixel scale is (100, 100) meters then to convert to point (center) it would be:

Move the Easting 50 meters to the east or right ( +.5 pixel)

Move the Northing 50 meters to the South or down ( -.5 pixel)

So doing ( -.5, -5 ) is wrong in a map projection.

We actually did a drawing of this on ossim.org to show the error when ignored but I cannot find it. Sorry.

Hope that helps,
Dave


On 05/13/2010 05:15 PM, Max Martinez wrote:
  GeoTIFF geographic grid alignment    
 

Tim,



I think the GeoTIFF spec tries to show through its examples that although the orientation of the positive J raster space axis is usually in the opposite direction of the positive Y model space axis, it is expected that both pixel scales will be positive under these conditions. (They probably could have been clearer if they described full map extents of the images in the examples). Negation should be used appropriately as the current condition differs from that usual condition. This is also supported by the example on page 27 where the ModelTransformationTag content equivalents of an image with tiepoint (I,J,K,X,Y,Z) and scale (Sx, Sy, Sz) is provided. So I think you have that right in your example.



We use a -0.5, -0.5 translation to translate pixel is area to pixel is point just has you have done below.



Max

 





From: geotiff-bounces@... [[hidden email]] On Behalf Of dburken@...
 Sent: Monday, May 10, 2010 1:48 PM
 To: tjn98
 Cc: Geotiff@...
 Subject: Re: [Geotiff] GeoTIFF geographic grid alignment

 


 

Hi,
 
If
RasterPixelIsPoint I think it would be (0.5, -0.5).
 
Dave
 
 
----- Original Message -----
From: "tjn98" <tim.nightingale@...> <[hidden email]>
To: Geotiff@...
Sent: Monday, May 10, 2010 10:28:07 AM GMT -05:00 US/Canada Eastern
Subject: [Geotiff] GeoTIFF geographic grid alignment
 
 
Dear All,
 
  I’m looking for some guidance on the correct interpretation of
GeoTIFF raster spaces, in particular their correct alignments at the
half-pixel level. Judging by the number of mutually contradictory
examples I’ve found, this is a common source of confusion.
 
  My interpretation of the rather terse text in the GeoTIFF Format
Specification (Revision 1.0) is that, for the example of a global
map measuring 129,600 longitude pixels by 64,800 latitude pixels,
a “PixelIsArea” raster described by:
 
    ModelTiepointTag = (0.0, 0.0, 0.0, -180.0, 90.0, 0.0)
    ModelPixelScaleTag = (0.002777777778, 0.002777777778, 0.0)
    GeoKeyDirectoryTag:
        GTModelTypeGeoKey    = 2    (ModelTypeGeographic)
        GTRasterTypeGeoKey   = 1    (RasterPixelIsArea)
        GeographicTypeGeoKey = 4326 (GCS_WGS_84)
 
exactly fits into a –180 -> 180 by –90 -> 90 degree box that touches
the edges of the extreme cell boundaries, and that a “PixelIsPoint”
raster, whose entries fall exactly in the centre of the “PixelIsArea”
cells in the first example is described by:
 
    ModelTiepointTag = (-0.5, -0.5, 0.0, -180.0, 90.0, 0.0)
    ModelPixelScaleTag = (0.002777777778, 0.002777777778, 0.0)
    GeoKeyDirectoryTag:
        GTModelTypeGeoKey    = 2    (ModelTypeGeographic)
        GTRasterTypeGeoKey   = 2    (RasterPixelIsPoint)
        GeographicTypeGeoKey = 4326 (GCS_WGS_84)
 
  Variants I have seen include both +0.5 and –0.5 pixel offsets for
the PixelIsArea case, negative ModelPixelScaleTag values for the
“J” direction and 0.0 pixel offsets for the PixelIsPoint case. QGIS,
for example, appears to assume the last.
 
  Can anyone help?
 
    Many thanks,
 
      Tim.
 
------------------------------------------------------------------------
 Dr T.J. Nightingale
  Space Science and Technology Department
   Rutherford Appleton Laboratory
    Chilton, Didcot                   Phone: +44/0 1235 445914
     Oxon OX11 0QX                    Fax:   +44/0 1235 445848
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Re: GeoTIFF geographic grid alignment

David Burken
In reply to this post by tjn98
Max,

I was looking at your stuff again and what I said was wrong for your case where you keep the tie geographic position the same you would use -.5, -5.

So:
    ModelTiepointTag = (0.0, 0.0, 0.0, -180.0, 90.0, 0.0)
    ModelPixelScaleTag = (0.002777777778, 0.002777777778, 0.0)
    GeoKeyDirectoryTag:
        GTModelTypeGeoKey    = 2    (ModelTypeGeographic)
        GTRasterTypeGeoKey   = 1    (RasterPixelIsArea)
        GeographicTypeGeoKey = 4326 (GCS_WGS_84)
And:
    ModelTiepointTag = (-0.5, -0.5, 0.0, -180.0, 90.0, 0.0)
    ModelPixelScaleTag = (0.002777777778, 0.002777777778, 0.0)
    GeoKeyDirectoryTag:
        GTModelTypeGeoKey    = 2    (ModelTypeGeographic)
        GTRasterTypeGeoKey   = 2    (RasterPixelIsPoint)
        GeographicTypeGeoKey = 4326 (GCS_WGS_84)

Are the same.

Typically we use (0, 0) with point but the geographic tie is relative to the center of the pixel not the edge.  So I apologize for the confusion.

Dave


On 05/10/2010 10:28 AM, tjn98 wrote:
GeoTIFF geographic grid alignment Dear All,

  I’m looking for some guidance on the correct interpretation of
GeoTIFF raster spaces, in particular their correct alignments at the
half-pixel level. Judging by the number of mutually contradictory
examples I’ve found, this is a common source of confusion.

  My interpretation of the rather terse text in the GeoTIFF Format
Specification (Revision 1.0) is that, for the example of a global
map measuring 129,600 longitude pixels by 64,800 latitude pixels,
a “PixelIsArea” raster described by:

    ModelTiepointTag = (0.0, 0.0, 0.0, -180.0, 90.0, 0.0)
    ModelPixelScaleTag = (0.002777777778, 0.002777777778, 0.0)
    GeoKeyDirectoryTag:
        GTModelTypeGeoKey    = 2    (ModelTypeGeographic)
        GTRasterTypeGeoKey   = 1    (RasterPixelIsArea)
        GeographicTypeGeoKey = 4326 (GCS_WGS_84)

exactly fits into a –180 -> 180 by –90 -> 90 degree box that touches
the edges of the extreme cell boundaries, and that a “PixelIsPoint”
raster, whose entries fall exactly in the centre of the “PixelIsArea”
cells in the first example is described by:

    ModelTiepointTag = (-0.5, -0.5, 0.0, -180.0, 90.0, 0.0)
    ModelPixelScaleTag = (0.002777777778, 0.002777777778, 0.0)
    GeoKeyDirectoryTag:
        GTModelTypeGeoKey    = 2    (ModelTypeGeographic)
        GTRasterTypeGeoKey   = 2    (RasterPixelIsPoint)
        GeographicTypeGeoKey = 4326 (GCS_WGS_84)

  Variants I have seen include both +0.5 and –0.5 pixel offsets for
the PixelIsArea case, negative ModelPixelScaleTag values for the
“J” direction and 0.0 pixel offsets for the PixelIsPoint case. QGIS,
for example, appears to assume the last.

  Can anyone help?

    Many thanks,

      Tim.

------------------------------------------------------------------------
 Dr T.J. Nightingale
  Space Science and Technology Department
   Rutherford Appleton Laboratory
    Chilton, Didcot                   Phone: +44/0 1235 445914
     Oxon OX11 0QX                    Fax:   +44/0 1235 445848
      United Kingdom                  Email: tim.nightingale@...
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